High voltage LED lighting has proven to be a viable replacement for older technologies such as High Intensity Discharge (HID) lighting. With the adoption of high voltage LED lighting, many manufacturers rushed to produce it and implement it in various applications. Although the quality of light and power density have increased considerably, efficiency has become an important aspect to consider. In addition, failure rates were much higher than expected in the first applications. The main challenge for high voltage LED lighting is to continue to increase power density and efficiency, as well as make it reliable and more affordable for future applications. This article will discuss Band Gap (GaN) technology and how it can solve the challenge of efficiency and power density in high voltage LED lighting. This discussion will show how bandgap technology can be used to maximize efficiency and power density, focusing on the step-down portion of the LED driver architecture shown in Figure 1.
Bandgap semiconductors (GaN) can operate at higher switching frequencies than conventional semiconductors such as silicon. Bandgap materials require a greater amount of energy to excite an electron from the top of the valence band to the bottom of the conduction band, where it can be used in the circuit. So increasing the bandgap has a big impact on a device (and allows a smaller die size to do the same job). Materials, such as gallium nitride (GaN), which have a higher bandgap, can withstand stronger electric fields. Critical attributes that bandgap materials have are high free electron velocities and higher electron field density. These key attributes make GaN switches up to 10 times faster and considerably smaller, with the same resistance and breakdown voltage as a similar component made of silicon. GaN is perfect for high voltage LED applications as these key attributes make it ideal for implementation in future lighting applications.
Figure 1: System architecture of a non-isolated high power LED driver. (Image source: STMicroelectronics)
Figure 1 shows a high-level architecture of an LED lighting application that will serve as a reference example for applying the GaN bandgap technology. Although bandgap materials can be used throughout the application, the high-voltage current generator, highlighted in green, will take center stage in leveraging bandgap technology to maximize efficiency and power density. Most lighting applications require high power factor and low harmonic distortion over a wide range of AC input voltage. In this case it is preferable to implement a PFC boost to provide a clean 400V inputCC for LED driver and meet power quality requirements. There are multiple options for a front end PFC boost converter; transition mode (TM), continuous driving mode (CCM), as well as others. The transition mode is characterized by variable frequency operation and zero current switching at power MOSFET turn-on. Other advantages are the simplicity of the design, the small size of the inductor, and the absence of reverse recovery of the boost diode. The main challenges are the high peak and RMS input current, which also results in a larger EMI filter as power increases. The CCM, on the other hand, provides fixed frequency operation. The pull-up inductor current always has a mean component, apart from near-zero crossing points. The inductor is designed for 20-30% ripple, resulting in a smaller EMI filter compared to TM operation. This also implies a larger boost inductor and smaller EMI filter for the same power output compared to TM operation. The main challenges are more complex control and the need for an ultra-fast soft recovery diode or SiC diode. Therefore, the CCM PFC is usually more expensive than a TM PFC. Ideally, a zero reverse recovery switch should be used instead of the rectifier diode in the CCM PFCs. This makes GaN transistors very good candidates for this application.
Isolation is optional and can be introduced between the input stage and the second power conversion stage. In this example, isolation is not used, and the input PFC stage is followed by a non-isolated reverse buck stage with CC/CV control. In cases where isolation is required, a resonant power converter (LLC, LCC) or a flyback converter can be used depending on the output power requirements of the application.
The PFC boost converter generates a regulated DC bus voltage at its output (higher than the peak of the input AC voltage) and passes this higher DC bus voltage to the inverted buck converter stage. The reduction operation is quite simple. When the buck switch is on, the inductor voltage is the difference between the input and output voltages (VIN - VOUT). When the switch is off, the catch diode rectifies the current and the inductor voltage is the same as the output.
MasterGaN System in Package (SiP) for LED Drivers
Along with power density and efficiency, a key challenge for high voltage lighting applications is design complexity. With the use of bandgap semiconductors such as GaN, the power density and efficiency of the circuit can be increased. ST's MasterGaN family addresses this challenge by combining high-voltage BCD gate drivers with high-voltage GaN transistors in a single package. MasterGaN allows easy implementation of the topology shown in Figure 1. It incorporates two 650 V GaN HEMT transistors in a half-bridge configuration, as well as the gate drivers. In this example, the entire buck power stage is built into a single 9x9mm QFN package that requires a minimal number of external components. Even the bootstrap diode, which is typically required to power the isolated high-voltage section of a high-side/low-side dual half-bridge gate driver, is built into the SiP. Consequently, the power density of an application using a MasterGAN device can be dramatically increased compared to a standard silicon solution, while increasing the switching frequency or power output. More specifically, in this LED driver application, a 30% reduction in PCB area was achieved and no heat sinks were used.
For high power LED lighting applications, CCM is the best mode of operation to use. When MCC is implemented with GaN devices, you get the high-level benefits discussed above, as well as reduced cost. There would be no need for an RDSON very low to serve high power applications due to the reduced contribution of switching losses to overall power losses. GaN also mitigates one of the main drawbacks of using CCM by eliminating recovery losses and reducing EMI, since GaN does not experience reverse recovery. CCM operation with fixed off-time control also makes it easy to compensate for the output current dependency on VOUT. It is clear that the implementation of GaN switches using CCM is a great match for high voltage LED lighting applications, as well as many others.
The basic schematic of a reverse buck topology is shown in Figure 2 along with an implementation using the MASTERGAN4.
Figure 2: Reverse buck topology implemented with MASTERGAN4. (Image source: STMicroelectronics)
MASTERGAN4 incorporates two 225mΩ (25°C typical) 650V GaN transistors in half-bridge configuration, a dedicated half-bridge gate driver, and the bootstrap diode. This high level of integration simplifies design and minimizes PCB area in a small 9x9mm QFN package. The evaluation board shown in Figure 3, designed with the MASTERGAN4 in a reverse buck topology, has the following specifications: it accepts up to 450V input, the LED string output voltage can be adjusted between 100V and 370V; operates on Fixed Time CCM (FOT) with a switching frequency of 70 kHz; the maximum output current is 1 A.
Figure 3: Demonstration example of reverse reducer with MASTERGaN4. (Image source: STMicroelectronics)
The controller in this solution, the HVLED002, is used to generate a single PWM control signal. An external circuit based on simple Schmitt triggers is then used to generate two complementary signals to drive the low-side and high-side GaN transistors with a suitable dead time. Two linear regulators are also included to generate the supply voltages required by the MASTERGAN4. The reverse buck topology implemented with MASTERGAN4 creates a solution to increase power density and efficiency, but let the results discussed below speak for themselves.
Experimental results:
The efficiency graphs in figure 4 show the advantages of the proposed solution compared to a traditional silicon solution as a function of the LED string voltage for output currents of 0.5 A and 1 A.
Figure 4: Efficiency versus voltage of the LED for the MasterGaN and Silicon MOSFET. (Image source: STMicroelectronics)
MASTERGAN4 efficiency remains at or above 96,8% across the entire voltage range of the LED string. It can be seen that at all power levels the efficiency gain is maximized thanks to the low conduction losses, as well as the minimal conduction and switching losses of the GaN solution.
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Table 1: GaN and Silicon MOSFET size comparison
Table 1 compares the silicon solution with the MASTERGAN4 based solution. As can be seen, a reduction of more than 30% of the total PCB area is shown with the implementation of the GaN design. The results show a path that can be taken with GaN in this reverse reducing topology. Increasing the switching frequency above 70 kHz can reduce the size of the output inductor and capacitor at the cost of higher conduction and switching losses. With a higher frequency and reduced filter size, electrolytic capacitors can be replaced with larger, more reliable ceramic capacitors. The balance between filter capacitor and buck inductor size can be optimized based on the switching frequency required by the target application.
Conclusions
This article discusses the implementation of a reverse buck topology for LED lighting applications based on MASTERGAN4. The system in package configuration has 650 V 225 mΩ GaN transistors in half-bridge configuration and dedicated gate drivers. The GaN vs. Silicon solution shows higher efficiency and smaller PCB surface area. MasterGaN is the ideal solution for a compact, high efficiency and power reverse buck implementation for lighting applications.
